Field of the Invention
The invention relates to a manufacturing method of carboxymethyl cellulose salt for the electrode of a nonaqueous electrolyte secondary battery.
Description of Related Art
In recent years, electronic machines, in particular portable devices such as mobile phones, personal digital assistants (PDAs), and notebook computers have become smaller, lighter, thinner, and more powerful, and portable devices are becoming more popular. As the application scope of such portable devices is diversified, the battery driving these portable devices has become a very important component. In a battery, the lithium ion secondary battery representative of the nonaqueous electrolyte secondary battery is extensively applied, and the lithium ion secondary battery has high energy density and high capacity.
In general, a nonaqueous electrolyte secondary battery is made by the following method. That is, the negative electrode and the positive electrode are respectively formed into thin films on the metal foil surface used as the current collector substrate (current collector) to form a thin film positive electrode and a thin film negative electrode, and the negative electrode contains a negative electrode active material formed by, for instance, a carbon material that can adsorb and release lithium ions, and the positive electrode contains a positive electrode active material formed by lithium-containing transition metal composite oxide (such as LiCoO2, LiNiO2, or LiMn2O4). Moreover, the thin film positive electrode and the thin film negative electrode are rolled or laminated with an isolation film that is also forming into a thin film between the two, and the rolled body or the laminated body is housed in the casing. The thin film positive electrode and the thin film negative electrode have the structures of a metal foil and mixture layer and are formed by coating and drying a negative electrode active material slurry (or paste) or a positive electrode active material slurry (or paste) on a current collector material, and the metal foil is a current collector substrate (current collector), and the mixture layer is formed on the metal foil surface and contains an active material.
Regarding the negative electrode, a method to conveniently manufacture the aqueous paste in which the aqueous paste does not contain a surfactant or a dispersant and only water-soluble polymer particles are present such that water-insoluble inorganic particles are evenly dispersed in the water and gel production is inhibited is as disclosed in patent literature 1. Specifically, after water-insoluble inorganic particles (in particular carbon material particles such as graphite) and water-soluble polymer particles (in particular particles of carboxymethyl cellulose or a salt thereof) are dry blended, the mixture is mixed with water to form an aqueous paste (in particular paste for the negative electrode material of a lithium ion battery), wherein the ratio of the average particle size of the water-insoluble inorganic particles and the average particle size of the water-soluble polymer particles is adjusted to former/latter=1/3 to 1/0.05 (in particular 1/2 to 1/0.1). According to the literature, the ratio of the average particle size of the water-insoluble inorganic particles and the average particle size of the water-soluble polymer particles is set to 1/3 to 1/0.05, and even if water-soluble polymer particles having higher viscosity (in particular carboxymethyl cellulose salt particles) are used, an aqueous paste inhibiting gel production and for which the particles are uniformly dispersed can still be made; the means of controlling the average particle size of the water-soluble polymer particles is a method in which the following pulverizers are used for pulverization, such as: sample mill, hammer mill, turbo mill, atomizer, cutter mill, bead mill, ball mill, roll mill, jet mill.
Moreover, the following carboxymethyl cellulose or a salt thereof is disclosed (patent literature 2) to prevent potential defects such as streaks and pinholes in the resulting electrode beforehand when used as the binder of the electrode of the nonaqueous electrolyte secondary battery. When 2 L of 0.3 mass % aqueous solution of the carboxymethyl cellulose or a salt thereof of dry mass B is first prepared, and the aqueous solution is completely filtered using a 250-mesh filter under a reduced pressure condition of −200 mmHg, and a dry mass A of the residue on the filter after filtering is measured, the ratio of dry mass A to dry mass B is less than 50 ppm. The literature recites that, if an aqueous solution of the current carboxymethyl cellulose or a salt thereof is prepared, then colloidal particles from the carboxymethyl cellulose or a salt thereof remain in the aqueous solution as undissolved substance, but mechanical dry or wet pulverization treatment is performed on the carboxymethyl cellulose or a salt thereof, and in the aqueous solution of the product of the mechanical pulverization treatment of the carboxymethyl cellulose or a salt thereof, the colloidal particles are micronized. The result is that, if an electrode is formed using the aqueous solution of the product of the mechanical pulverization treatment of the carboxymethyl cellulose or a salt thereof, then large undissolved substance can be inhibited, wherein the undissolved substance causes streak defects, peeling, or pinholes . . . etc. on the electrode surface.    Patent literature 1: JP 2011-63673    Patent literature 2: JP 5514734